The present invention is directed to a process for producing substantially dry water insoluble polymers from poly alkyl esters of vinylic monomers for improved flocculation of red mud in the Bayer process and for a process of clarification of the red mud-containing liquors generated in the Bayer process for the recovery of alumina from bauxite ore with high molecular weight emulsion polymers.
The Bayer process for the production of alumina includes the basic steps of pulverizing a bauxite ore which is slurried in water and digested with caustic at elevated temperatures and pressures. The caustic solution dissolves oxides of aluminum to form an aqueous sodium aluminate solution that must be separated from the caustic-insolubles of bauxite ore (red mud). Separation typically occurs through the process of sedimentation and filtration, which are often aided by a flocculant. Once separated, alumina trihydrate can be precipitated out of the aqueous sodium aluminate solution and collected as product.
Specifically, the pulverized bauxite ore is fed into a slurry mixer where the slurry makeup water is typically spent liquor and added caustic. The bauxite ore slurry is then diluted and passed through a digester under high pressure and temperature conditions where about 98% of the total available alumina is released from the ore as caustic-soluble sodium aluminate. After digestion, the slurry passes through several flash tanks where the pressure of the digested slurry is reduced from several atmospheres to one atmosphere and the temperature of the slurry is reduced from about 200xc2x0 C. to about 105xc2x0 C.
The aluminate liquor leaving the flashing operation contains about 1 to 20 weight percent solids. The coarser solids may be removed from the aluminate liquor with xe2x80x9csand trapxe2x80x9d cyclones. The finer solids are generally separated from the liquor first by settling aided by a flocculant and then filtration. The aluminate liquor slurry leaving the flash tank is diluted by a stream of recycled washer overflow liquor. Any Bayer process slurry taken from the digesters through a subsequent dilution of the slurry, including the flash tanks, but before the primary settler, is the primary settler feed.
Normally, the primary settler feed is fed to the primary settler where it is treated with a flocculant. As the mud settles, the clarified sodium aluminate solution (xe2x80x9cgreenxe2x80x9d or xe2x80x9cpregnantxe2x80x9d liquor) overflows to a weir and is collected. This overflow from the primary settling tank is then passed through subsequent process steps.
The clarity of the primary settler overflow is crucial to efficient processing of alumina trihydrate because further steps will be required if the overflow has not been properly clarified. An overflow liquor containing a concentration of suspended solids of about 10-500 mg suspended solids per liter is typically unacceptable and will require further processing in a secondary clarification stage to achieve the desired 10 mg suspended solids per liter.
A flocculant is often added to aid in the partial separation of the red mud solids from the pregnant liquor in the primary settler stage. Flocculating agents, such as inverse emulsion liquid polymers, dry polymers and polysaccharides including starch, improve the separation of insolubles. Flocculants are able to increase the rate at which solids settle by reducing the amount of residual solids suspended in the liquor and by decreasing the amount of liquor in the settled solids phase.
Flocculants are highly important in the primary settling stages. Red muds are comprised of iron oxides (about 50 weight percent), together with silicon oxides, calcium oxides, sodium alumino-silicates, titanium oxides and other material commonly comprise about 5 to about 50 weight percent of the bauxite ore. Generally, the muds are comprised of very fine particles, which hinder separation of red mud particles from the solubilized alumina liquor. If the rate of separation is too slow, output is diminished and the overall process efficiency is impaired. If the separation is not clean, a more extensive treatment to remove residual solids will be required.
Conventional treatments include the application of polysaccharides, such as starch and dextran. For instance, U.S. Pat. No. 3,085,853 to Lesinski et al. uses native dextrans to increase the rate of sedimentation of finely divided solids in aqueous suspensions. After polysaccharides were introduced, synthetic polymeric flocculants were developed which have became more popular for use in the Bayer process. Additional polymers include those formed from the polymerization of vinyl amine or vinyl formamide as disclosed in U.S. Pat. No. 5,346,628; phosphonic acid polymers as disclosed in U.S. Pat. No. 5,534,235; and polymers containing hydroxamic acid groups in U.S. Pat. No. 4,767,540.
Other Bayer process treatments include the combination of polysaccharides with synthetic polymeric flocculants. For example, U.S. Pat. No. 3,397,953 to Galvin et al. discloses that polyacrylic acid alone is not suitable as a flocculating agent, instead it teaches the use of a blend of starch and polyacrylic acid on red mud suspensions. The polyacrylic acids generally have molecular weights of less than 300,000. The flocculation and sedimentation activity of the blend is used in the primary settler stage of a bauxite process, and contains at least about 80 weight percent of the acrylic acid mer with a molecular weight in excess of 50,000 and preferably in excess of 100,000. Other starch/polymer combinations are disclosed in U.S. Pat. Nos. 3,397,953; 3,541,009; 3,681,012; and 5,008,089.
Emulsions of polymers formed of lower primary alkyl acrylate monomers for refining alumina are disclosed in U.S. Pat. No. 3,755,531. Moreover, unexamined Japanese Pat. No. 4986-1979 discloses a process for producing an ultra high molecular weight acrylic acid copolymer salt. However, only lower molecular weight polymers which are less effective flocculants than higher weight polymers are disclosed in the Japanese Patent.
Disclosed in U.S. Pat. No. 5,286,391 is a process for improving the flocculation of suspended red mud solids in a Bayer process liquor comprising a water-continuous dispersion containing a polymer dispersed in the water of the continuous phase, wherein the polymer comprises at least 50 mole percent of mer units having pendant groups that hydrolyze to pendant carboxylic acid groups. However, this reference suggests that because the in-situ hydrolysis occurs over time and results in continuous activation, that the optimal polymer would be one containing 100 mole percent of mer units having pendant hydrolyzable groups. By contrast, this invention discloses that there is an optimal degree of hydrolysis (not 100%), that occurs in the range of 70-80%.
Oil continuous liquid flocculants (hereafter termed o/c flocculants) and powder flocculants are typically referred to as synthetic flocculants. Such flocculating agents are added to the primary settler, the mud washing circuit, mud-dewatering systems including centrifuges and vacuum filters and to enhance mud stacking in mud disposal sites.
High molecular weight synthetic polymers are often used in conjunction with starches. Starch is said to improve supernatant liquor clarity, the interface between the settled mud and the cleaner supernatant liquor, and reportedly helps to maintain the stability of the settled mud in settlers and washers. However, starch occasionally produces a red mud which has poor rheology and undesirable underflow characteristics. If not diluted, the underflow can occasionally become difficult to pump.
The oil continuous flocculants must be inverted prior to application to transfer the flocculant to the water phase of the emulsion. If o/c flocculants are added neat (non-inverted) the polymer flocculant will not disperse in the Bayer liquor, the oil phase containing the flocculant will plug the injection equipment, and poor activity will be encountered.
Oil continuous liquid flocculants of sodium or ammonium acrylate are largely used in the bauxite industry for settling the red mud. Since the oil continuous liquid flocculants contain about 25-30% of paraffinic oils as the carrier, one drawback to these flocculants is the liquor from which the alumina separates out becomes enriched with increasing amounts of organic material. Since organics in the liquor inhibit the precipitation of alumina, a flocculant containing the least amount of organic material is the most desirable. The liquid flocculants are generally easier to transfer, store and apply to the process than dry powders. However, shipping costs for the liquids tend to be considerably higher since the emulsions contain only about 25-40% polymer solids. Furthermore, some o/c liquids can become unstable if exposed to temperatures below their freezing point.
Dry polymers are an effective alternative to liquid polymers in the flocculation of Bayer process liquors. As opposed to emulsion polymers, dry polymers contribute little to the organic load of the Bayer liquor and are less expensive to ship. While such advantages have made the use of dry polymers more common, they remain under used because of the special handling required to ensure the polymers remain dry before use. Dry polymers are extremely hygroscopic and must be handled, stored and prepared without exposing the polymer to moisture or humid air. Once a dry polymer has been exposed to moisture or humidity, severe system plugging and loss of activity will result. The hydration of a dry polymer prior to its use results in product loss, disposal problems, and increased maintenance costs. Dry polymer storage, solution make-up and feeding equipment must be very elaborate to prevent hydration.
Additionally, dry polymers are difficult to dissolve in water, with 90%+polymer hydration requiring mixing and aging time as high as 2 hours. This slow dissolution rate requires large inventories of solutions divided into 2 separate tanks. The first tank is the high mixing energy tank where dissolution occurs. The second tank is the working solution tank from which the polymer solution is drawn and fed into the process.
Accordingly, it would be advantageous to provide a substantially dry flocculating agent which would expedite the separation of the red mud solids from the pregnant liquor, improve the clarity of liquor overflow, be easy to handle and feed into the process without the hygroscopic characteristics of current dry polymers.
The present invention is directed to a method for producing substantially dry water insoluble polymers from poly alkyl esters of vinylic monomers for use in a method for the clarification of red mud-containing liquors generated in the Bayer process for the recovery of alumina from bauxite ore. The polymers can be either homopolymers, copolymers or terpolymers. Preferably, the dried emulsion polymers are high molecular weight polymers of poly(methyl acrylate), poly(methyl acrylate/acrylic acid), poly(vinyl acetate/acrylic acid/methyl acrylate), poly(t-butyl methacrylate/acrylic acid/methyl acrylate) and poly(t-butyl acrylate/acrylic acid/methyl acrylate).
One aspect of this invention is a process for producing substantially dry water insoluble high molecular weight polymers from poly alkyl esters of vinylic monomers comprising:
spray-drying into a gas stream an aqueous dispersion of a high molecular weight water-continuous emulsion polymer including an emulsifier, a surfactant, water and having an intrinsic viscosity in 2N NaNO3 at 30xc2x0 C. of at least 15 dL/g; and
collecting the resultant polymer particles.
A second aspect of the present invention includes a process for producing substantially dry water insoluble high molecular weight polymers from poly alkyl esters of vinylic monomers comprising:
drying an aqueous dispersion of a high molecular weight water-continuous emulsion polymer including an emulsifier, a surfactant, water and having an intrinsic viscosity in 2N NaNO3 at 30xc2x0 C. of at least 15 dL/g;
forming a film comprised of the dried aqueous dispersion substantially free of water; and
grinding the film to form a powder of substantially dry polymer particles.
An additional aspect of this invention is a method of treating Bayer process red mud containing liquor comprising the steps of:
adding to the red mud containing liquor a substantially dry water insoluble high molecular weight polymer formed from a polymer emulsion of poly alkyl esters of vinylic monomers having an intrinsic viscosity in 2N NaNO3 at 30xc2x0 C. of at least 18 dL/g;
forming a red mud phase and a clarified liquor phase; and
recovering the clarified liquor phase.
Another aspect of this invention is a method for treating Bayer process red mud containing liquor comprising the steps of:
adding to the red mud containing liquor an effective amount of a substantially dry water insoluble high molecular weight polymer formed from a homopolymer emulsion wherein the homopolymer has an intrinsic viscosity in 2N NaNO3 at 30xc2x0 C. of at least 15 dL/g;
forming a red mud phase and a clarified liquor phase; and
recovering the clarified liquor phase.
A further aspect of this invention is a method for treating Bayer process red mud containing liquor comprising the steps of:
adding to the red-mud containing liquor an effective clarifying amount of a substantially dry water insoluble high molecular weight polymer formed from a terpolymer emulsion wherein the terpolymer has an intrinsic viscosity in 2N NaNO3 at 30xc2x0 C. of at least 15 dL/g;
forming a red mud phase and a clarified liquor phase; and
recovering the clarified liquor phase.